Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

LIQUEFACTION 50 YEARS AFTER

ANCHORAGE 1964; HOW

NANOPARTICLES COULD PREVENT IT

F. Ochoa-Cornejo1, A. Bobet

2, C.T. Johnston

3, M. Santagata

4 and J.V.

Sinfield4

ABSTRACT

Earthquake induced liquefaction of loose sand deposits has been responsible for significant

damage in severe seismic events (e.g. Anchorage 1964, Chile 2010, Japan 2011); therefore, there

is a need for developing soil improvement methods to increase the liquefaction resistance of

these deposits, particularly in proximity to existing structures, where traditional approaches

relying on densification may not always be used. This paper presents an experimental study that

explores the use of laponite a synthetic nano-clay with particle diameter ten times smaller than

bentonite for treating liquefiable soils. The effect of the presence of a small amount of laponite

(1% by dry mass of the sand) on the liquefaction resistance of clean sand specimens with relative

density in the 15-25% range is studied through laboratory cyclic tests. It is found that the

addition of 1% laponite leads to a significant increase in liquefaction resistance, with respect to

the clean sand, with the number of cycles to liquefaction increasing by approximately two orders

of magnitude under the same cyclic stress ratio. This is comparable to the effect recently reported

for bentonite, but with much smaller addition of the nano-clay. The study addresses the

mechanisms that explain the improvement based on the thixotropic fluid formed in the pore

space.

1 Graduate Student Researcher, School of Civil Engineering, Purdue University, West Lafayette, IN 47906

2 Professor, School of Civil Engineering, Purdue University, West Lafayette, IN 47906

3 Professor, Department of Agronomy, Purdue University, West Lafayette, IN 47906

4 Associate Professor, School of Civil Engineering, Purdue University, West Lafayette, IN 47906

Ochoa-Cornejo F., Bobet A., Johnston C., Santagata M., Sinfield J. Liquefaction 50 years after Anchorage 1964;

how nanoparticles could prevent it. Proceedings of the 10th

National Conference in Earthquake Engineering,

Earthquake Engineering Research Institute, Anchorage, AK, 2014.

DOI: 10.4231/D3CC0TT8W

LIQUEFACTION 50 YEARS AFTER ANCHORAGE 1964; HOW

NANOPARTICLES COULD HELP PREVENT IT

F. Ochoa-Cornejo2, A. Bobet

2, C.T. Johnston

3, M. Santagata

4 and J.V. Sinfield

4

ABSTRACT Earthquake induced liquefaction of loose sand deposits has been responsible for significant

damage in severe seismic events (e.g. Anchorage 1964, Chile 2010, Japan 2011); therefore, there

is a need for developing soil improvement methods to increase the liquefaction resistance of these

deposits, particularly in proximity to existing structures, where traditional approaches relying on

densification may not always be used. This paper presents an experimental study that explores the

use of laponite a synthetic nano-clay with particle diameter ten times smaller than bentonite for

treating liquefiable soils. The effect of the presence of a small amount of laponite (1% by dry

mass of the sand) on the liquefaction resistance of clean sand specimens with relative density in

the 15-25% range is studied through laboratory cyclic tests. It is found that the addition of 1%

laponite leads to a significant increase in liquefaction resistance, with respect to the clean sand,

with the number of cycles to liquefaction increasing by approximately two orders of magnitude

under the same cyclic stress ratio. This is comparable to the effect recently reported for bentonite,

but with much smaller addition of the nano-clay. The study addresses the mechanisms that explain

the improvement based on the thixotropic fluid formed in the pore space.

Introduction

Liquefaction is a devastating phenomenon observed during earthquakes in saturated loose

granular deposits subjected to undrained cyclic loading. In such deposits, during earthquake

shaking, the pore water pressure increases causing the effective stress of the soil to decrease.

Once the pore pressure equals the initial effective confinement, liquefaction is triggered causing

large deformations in the soil. The Great Alaska Earthquake in 1964 provided remarkable

examples of the extent of damage that liquefaction can cause. The earthquake that hit Anchorage,

with magnitude Mw = 9.2, caused the settlement and collapse of large structures built on off-

shore saturated loose sand deposits, along the shoreline and in the interior of the State. Due to the

damage observed in this and other earthquakes (e.g. El Maule 2010 in Chile and Tohoku 2011 in

Japan), efforts have increased to improve and develop effective methods to strengthen the soil

resistance to liquefaction, in particular in proximity to existing structures (e.g. see the work with

colloidal silica, [1]), where more traditional soil improvement solutions, relying on densification,

can often not be employed. One method that has recently shown promise, relies on modifying the

1 Graduate Student Researcher, School of Civil Engineering, Purdue University, West Lafayette, IN 47906

2 Professor, School of Civil Engineering, Purdue University, West Lafayette, IN 47906

3 Professor, Department of Agronomy, Purdue University, West Lafayette, IN 47906

4 Associate Professor, School of Civil Engineering, Purdue University, West Lafayette, IN 47906

Ochoa-Cornejo F., Bobet A., Johnston C.T., Santagata M., Sinfield J.V. Liquefaction 50 years after Anchorage

1964; how nanoparticles could prevent it. Proceedings of the 10th

National Conference in Earthquake Engineering,

Earthquake Engineering Research Institute, Anchorage, AK, 2014.

pore fluid of the sand, replacing, through permeation, the pore water with a concentrated

bentonite suspension [2,3]. The idea for this approach to soil improvement for liquefaction

mitigation comes from the well documented effect that the presence of plastic fines has on the

liquefaction resistance of sands. Both laboratory results and field observations have shown that

the presence of plastic fines in the sand matrix prevents liquefaction or increases the number of

cycles to liquefaction, and that the plasticity of the fines plays an important role [4,5,6,7,8,9].

This effect is recognized in the approach recommended for deriving liquefaction resistance from

the normalized SPT number shown in Fig. 1. This chart shows that the liquefaction resistance of

sands increases with the fines content.

Figure 1. Influence of fines content on the liquefaction of sands [9].

The positive effect that the presence of highly plastic fines has on the liquefaction resistance of

sands is clearly documented by El Mohtar et al. [2], who, based on cyclic triaxial tests, show that

the addition of 3% of bentonite by dry mass of sand is sufficient to increase the number of cycles

to liquefaction by at least an order of magnitude relative to clean sand specimens of the same

skeleton relative density. While bentonite has distinct advantages because it is a natural,

environmentally safe product, and because of the existing knowledge base in geotechnical

engineering, its use in this particular application presents some significant challenges. First, the

delivery through permeation of a concentrated bentonite suspension into the sand pore space

requires chemical treatment using sodium pyro-phosphate of the clay (see more below) to

modify its short-term rheology [10]. Second, permeation of the bentonite may not be possible in

finer sand deposits. Finally, impurities and batch-to-batch differences in the supplied bentonite

can create difficulties in consistently reaching the desired results. These difficulties can be

overcome by using laponite, a synthetic nano-particle with a diameter ten times smaller than

bentonite and a naturally delayed gelation process [11]. These characteristics suggest that it

could be used to treat even fine sand deposits, without requiring chemical modification.

Additionally, laponite has a plasticity index of 1200, almost twice that of bentonite, suggesting

that smaller percentages may be required relative to bentonite to achieve the same improvement

in liquefaction resistance.

This is the premise for the study presented in this paper, which quantifies the impact of

the addition of 1% laponite (by dry mass on the sand) on the cyclic response of clean Ottawa

sand. Consistent with previous work performed by the authors with bentonite, testing relies on

specimens obtained by mixing the laponite with the sand under dry conditions prior to specimen

preparation (“wished in place” laponite). The results are compared to previously published data

collected by the authors for sand-bentonite specimens.

Experimental Program

Materials

The materials used to prepare the specimens tested in this study are clean Ottawa sand (C778)

and laponite powder. Ottawa sand is a clean silicate sand. Sand particles have a soft grey color

with a rounded to sub-rounded shape. The coefficient of uniformity Cu is 1.48; its D50 is 0.4 mm,

with maximum and minimum void ratios emax = 0.48 and emin = 0.78. The specific gravity, Gs, is

2.65. Laponite RD is a synthetic smectite clay, with 1 nm thickness, and diameter of

approximately 25 nm diameter, i.e. one order of magnitude smaller than bentonite [12]. The 2:1

structure of laponite is comprised of a dioctahedral sheet sandwiched between two silicon

tetrahedral sheets. Partial replacement of magnesium ions by lithium ions in the octahedral sheet

results is a particle with negatively charged faces (while the edges have small localized positive

charges). Fig. 2 shows the structure and chemical composition of bentonite and laponite, and

compares their particle size.

Figure 2. Laponite and bentonite (Modified from [13,10]).

The specific gravity of laponite is 2.57, compared to 2.65, reported for bentonite. The specific

surface area of laponite determined from water vapor sorption experiments using the BET model

is 470 m2g [13], comparable to values determined for bentonite using the same methodology

(e.g. 440 m2g [14]).

Laponite suspensions have very characteristics rheological properties: for concentrations

up to approximately 3% by mass of water, they initially behave as Newtonian fluids; however,

after a few hours a “solid-like” gel structure [15] able to resist shear stresses, and with

thixotropic properties [16] is formed. This transition from sol to gel is reflected in the increase in

storage modulus (G’) shown in Fig. 3 for a 3% laponite suspension. G’, which is obtained from

small strain oscillatory measurements, is a measure of the elastic component of the material

response. The data for 4% laponite in Fig. 3 also show an increase in G’. However, in this case

the initial behavior of the suspension is not Newtonian. Fig. 3 also presents data of G’ versus

time for two bentonite suspensions: one with 10% bentonite and the other with 10% bentonite

and 0.5% sodium pyrophosphate (SPP). SPP is used to delay the formation of the bentonite gel,

to allow delivery of the bentonite in a porous medium, as described by El Mohtar et al. [2]. Note

that a 10% concentration by mass of water translates into approximately 3% bentonite by dry

mass of the sand, once the suspension is delivered into a sand matrix. Figure 3 also shows that,

after sufficient time for gelation, the two laponite suspensions can be viewed as rheologically

equivalent (in terms of G’) to the 10% bentonite suspension. This is significant as it has been

shown that the delivery of a 10% bentonite suspension inside the pores of a sand, is successful in

preventing the liquefaction of the treated sand [2]. Thus, the experimental program with laponite

presented in this paper focuses on the investigation of the cyclic resistance of Ottawa sand

specimens mixed with laponite such that a 3% laponite suspension is formed in the pore space of

the sand. Note that such concentration corresponds to specimens prepared with 1% laponite with

respect to the dry mass of the clean sand.

Figure 3. Storage modulus with time of laponite and bentonite suspensions

(bentonite data from [2])

Specimen Preparation and Testing

Specimens are prepared by dry mixing clean sand with 1% of laponite powder (by dry mass of

sand) in a plastic cylinder equipped with a pluviation tube. The mixture is then pluviated in the

split mold set up on the triaxial base (two different triaxial apparatuses are used in this research:

the CKC system manufactured by Soil Testing Equipment of San Francisco, CA, and the STX-

050 system manufactured by GCTS of Tempe, AZ), controlling the drop height to achieve a

skeleton relative density in the 15-25% range. The method is highly repeatable and produces

uniform specimens. The skeleton relative density is calculated using the skeleton void ratio,

which considers the fine content as part of the void space and utilizes the values of emax and emin

of the clean sand. Note that the use of a constant skeleton void ratio allows comparisons between

the clean sand, and the same sand in which part of the pore space is occupied by laponite fines.

Following the application of an initial isotropic cell pressure of 25 to 50 kPa, the specimens are

flushed first with carbon dioxide (CO2) and then with deionized deaired water. A backpressure

between 200 kPa and 300 kPa is then applied to ensure saturation while maintaining the effective

stress constant. A minimum B-value of 0.95 is considered acceptable to increase the

consolidation stress to 100 kPa. Following consolidation, the clean sand specimens are

immediately subjected to the undrained cyclic shear, while the sand-laponite specimens are

allowed to age 72 hours. Note that 72 hours corresponds to the time in Fig. 3 when the storage

modulus of the 3% laponite suspension reaches a plateau, which is taken as an indication that

gelling of the laponite suspension is complete (any additional increase in G’ can be attributed to

aging). Note also that at this time, the storage modulus of the laponite suspension is comparable

to that of the 10% bentonite suspension, which has been proven to be effective in increasing the

liquefaction resistance of the sand [2]. Cyclic shear is performed with a loading frequency of 1

Hz. Cyclic stress ratios (where CSR is the ratio between the applied cyclic shear and the

effective initial confinement) ranging between 0.1 and 0.14 are applied for clean sand and

between 0.12 and 0.25 for sand-laponite specimens. The tests are run undrained until liquefaction

occurs. In this paper, liquefaction is defined in correspondence to the complete loss of the

effective confining stress in the specimen.

The procedures described are similar to those followed in [2,3] to prepare dry-mixed

specimens of bentonite and Ottawa sand. The differences between the two procedures lie in the

amount of clay dry-mixed with the sand (3% bentonite versus 1% laponite) and the pre-shear

ageing duration (72 hours for the sand-laponite specimens versus either 24 or 96 hours for the

sand bentonite specimens). Another important difference is in the value of the skeleton relative

density targeted in the two studies: 15-25% for the sand-laponite specimens, versus 35-45% for

the sand-bentonite specimens examined in [2,3].

Cyclic Triaxial Tests Results

Figs. 4 and 5 show the response of specimens of clean sand and sand-laponite specimens during

the undrained cyclic stage of triaxial tests. Figs. 4(a) and 5(a) are plots of the applied deviatoric

stress (equal in this case to the vertical stress deviator ) with the number of cycles (N); Figs.

4(b) and 5(b) show the axial strain, Figs. 4(c) and 5(c) the excess pore pressures and Figs. 4(d)

and 5(d) the stress path in terms of p’- q (p’= [’v+’h q=[v-’h), all with respect to the

number of cycles. Both tests start from an initial effective confinement of 100 kPa.

Figs. 4(c) and 5(c) show that liquefaction occurs in both tests. However, in the case of the

sand-laponite specimen, despite the marginally lower value of the skeleton relative density, and

the higher CSR applied (0.135 versus 0.085), a significant increase in number of cycles to

liquefaction (from 125 to over 500) is observed. (Note that as shown in Fig. 6, under a CSR of

0.138, the clean sand specimen would be able to sustain only 1-2 loading cycles). Despite the

difference in number of cycles to liquefaction, the development of excess pore pressure shows a

similar trend in both specimens: the excess pore pressure shows an initial very rapid increase,

until a “plateau” is reached in which the rate of excess pore pressure generation is approximately

constant; at some point, the rate of excess pore pressure generation accelerates sharply and

liquefaction is reached after a few cycles.

Figure 4. Undrained cyclic response of clean sand

Figure 5. Undrained cyclic response of sand specimen with 1% laponite

Comparison of Fig. 4(c) and Fig. 5(c) shows that the presence of just 1% laponite affects all

stages of excess pore pressure generation, reducing the rate of pore pressure generated in the

plateau, extending the plateau itself, and allowing the specimen to sustain more cycles after the

point of acceleration and before liquefaction. This delay in pore pressure generation leads to an

increase in the cyclic resistance of the sand. The effective stress paths followed by the specimens

during testing (Figs. 4(d) and 5(d)) reflect the increase in pore pressure with the number of

cycles. With increasing number of cycles, the stress path moves to the left, eventually

intersecting the failure envelope of the soil, at which time the imposed deviatoric stress cannot be

sustained by the specimen and begins to decrease, as shown in Figs. 4(a) and 5(a). The specimen

deformations, depicted by the axial strain plots in Figs. 4(b) and 5(b) indicate very small strains,

for most of the cycles applied due to the fact that the undrained loading conditions prevent

significant changes in volume, until very close to the point of liquefaction, at which point a rapid

increase in the measured strains is observed. Similar observations on the effect of the addition of

small percentages of bentonite on the excess pore pressure generation behavior and axial strain

development are reported by El Mohtar et al. [2,3] based on tests on sand specimens with 3%

bentonite.

Fig. 6 compiles the results of several tests conducted on clean sand and sand-laponite

specimens, all with skeleton relative density in the 15-25% range, by plotting the number of

cycles to liquefaction versus the applied CSR. The arrow in the figure indicates that the specimen

did not liquefy at the number of cycles shown, at which point the test was terminated. The figure

clearly highlights the improvement attained in the response of clean sand following treatment

with 1% laponite: that is, for the same CSR applied, the number of cycles to reach liquefaction

increases by over two orders of magnitude.

Figure 6. Cyclic resistance of clean sand and sand-laponite specimens with Drsk~15-25%.

Fig. 7 plots similar results for clean sand specimens and sand specimens with 3% bentonite, all

with skeleton relative density in the 30-40% range. Two sets of data for sand-bentonite

specimens are included in the figure: one for specimens with a 24 hour post-consolidation ageing

period, the other for an aging duration of 96 hr. These two data sets are shown because they

bracket the 72 hr aging time used for the sand-laponite specimens. The figure indicates that as in

the case of laponite, the addition of bentonite increases the cyclic resistance of the treated sand

by one order of magnitude relative to the clean sand. Moreover, an increase in aging time

produces a further increase in resistance.

Figure 7. Cyclic resistance of clean sand and sand-bentonite specimens with Drsk~30-40% [2].

Figs. 6 and 7 clearly demonstrate that the presence of plastic fines inside a sand matrix has a

positive effect on the liquefaction resistance of the sand. They also suggests that the plasticity of

the fines plays an important role in the cyclic response of the soil, since the addition of 1%

laponite accomplishes an improvement similar to that achieved with 3% bentonite.

The improved liquefaction resistance observed with the addition of a highly plastic nano-

clay is caused by a delay of the excess pore pressure generation during cyclic loading. It is

hypothesized that this delay is the result of the interaction between the sand grains and the solid-

like pore fluid formed by the plastic fines and the water in the pore space, which reduces the

mobility of the sand grains, delaying the excess pore pressure generation, and thus increasing the

cyclic resistance of the soil.

This hypothesis is supported by direct observation of the microstructure of sand-bentonite

and sand-laponite specimens using cryo-scanning electron microscopy (cryo-SEM). This

technique involves sublimation of the samples at very low temperatures (-85 °C) and imaging

under cryogenic conditions (-130 °C), so that the sample remains close to its natural state and

dehydration is avoided (for details see [17]). Fig. 8 presents images of the microstructure of a

sand-bentonite specimen (Fig.8a) and a sand-laponite specimen (Fig. 8b). Both images show that

the sand grains are surrounded by a dense cellular matrix that corresponds to the gel formed in

the pore space from hydration of the bentonite or the laponite. This pore fluid is characterized by

solid-like behavior and properties (e.g. the storage modulus depicted in Fig. 2) that increase the

elastic response of the sand, preventing or at least reducing irrecoverable plastic deformations,

and thus reducing the generation of excess pore pressures.

Figure 8. Cryo-SEM photographs of: (a) sand-bentonite; and (b) sand-laponite specimens [13].

Conclusions

The paper presents the results of an experimental program carried out on dry-mixed specimens of

sand and 1% laponite. The goal of the program was to evaluate the impact of the addition of 1%

laponite on the undrained cyclic triaxial response of the sand and compare the results to those for

clean sand and sand treated with bentonite. The results show that the presence of 1% laponite

significantly increases the number of cycles to liquefaction, compared to the clean sand tested

under the same conditions (same Drskeleton and CSR). The improvement is comparable to that

obtained using bentonite. However, consistent with the greater plasticity of laponite relative to

bentonite, less laponite appears necessary to achieve the same degree of improvement. The

increase in cyclic resistance is attributed to the gel formed inside the pore space as a result of the

hydration of the laponite. This thixotropic fluid with solid-like behavior reduces the mobility of

the sand particles, thus effectively delaying the generation of excess pore pressures, resulting in

an increase of the liquefaction resistance. Overall, the experimental program provides

encouraging evidence in support of the use of laponite for liquefaction mitigation.

Acknowledgements

The authors gratefully acknowledge the support of the National Science Foundation for funding

this research, under grant number 0928679. Special thanks go to Mr. Alain El Howayek for

sharing the cryo-SEM pictures.

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